The present invention is directed to probe equipment for low noise measurements.
It is sometimes desirable to determine the functional characteristics and/or the performance of an electrical device, such as an integrated circuit. One of the performance characteristics of interest for an electrical device is the noise level of the device itself or a combination of devices. The noise contribution of each device in a signal path should be sufficiently low that collectively they do not interfere with the ability to detect the signals. Typically, the level of the noise is referred to as the signal to noise ratio of the device. In some cases, the level of the noise is relative to a known potential, such as ground.
The level of the noise of a device, such as the noise observed at the collector or drain of a device, is used to describe the characteristics of the device or collection of devices. For example, an ideal amplifier would produce no noise of its own, but simply amplify the input to an output level. By way of example, a 10 dB amplifier would amplify the signal, including any noise included therein, at its input by 10 dB. Therefore, although the noise would be 10 dB higher at the output of the amplifier, the overall signal to noise ratio would remain unchanged. However, the device itself may produce additional noise, that when added to the input noise, results in a decrease in the signal-to-noise ratio. Accordingly, to lower the overall noise levels it is desirable to reduce the amount of noise added by the device itself.
Thermal noise, generally referred to as Johnson noise, is generated by the thermal agitation of electrons in a conductive material. In general, this noise is produced by the thermal agitation of the charges in an electrical conductor and is proportional to the absolute temperature of the conductor. It tends to manifest itself in the input circuits of devices such as amplifiers where the signal levels are low. The thermal noise level tends to limit the minimum noise that a circuit can attain at a given temperature. The thermal noise tends to be relatively uniform throughout the frequency spectrum and depends on k (Boltzman constant) and T (temperature in degrees Kelvin).
Shot noise is typically generated where there is a potential barrier (voltage differential). One example of such a potential barrier is a p-type/n-type junction diode. Shot noise is generated when the electrons and holes cross the barrier. On the other hand, a resistor normally does not produce shot noise since there is no potential barrier within a resistor. Current flowing through a resistor does not typically exhibit such fluctuations. However, current flowing through a diode and similar devices produces small signal fluctuations. This is due to electrons (in turn, the charge) arriving in quanta, one electron at a time. Thus, the current flow is not continuous, but limited by the quantum of the electron charges.
1/f (one-over-f) noise, generally referred to as flicker noise, is found in many natural phenomena such as nuclear radiation, electron flow through a conductor, or even in the environment. Flicker noise is associated with crystal surface defects in semiconductors. The noise power tends to be proportional to the bias current and, unlike thermal noise and shot noise, flicker noise decreases with frequency. An exact mathematical model for flicker noise does not exist because it tends to be device specific. However, flicker noise tends to exhibit an inverse proportionality with frequency that is generally 1/f for low frequencies. Flicker noise tends to be essentially random in nature, but, because its frequency spectrum is not flat, it is not considered a true white noise. In general, flicker noise tends to have the characteristic that the longer the time spent measuring flicker noise, the greater the fluctuation in the measurements. Likewise, the less the time spent measuring flicker noise, the less the fluctuation in the measurements. Time is related to 1/frequency, so this flicker noise has been named 1/f noise
Flicker noise tends to be more prominent in FETs, and bulk carbon resistors. Flicker noise is present in many other types of devices, including for example, MOSFETs, CMOS, bipolar junction transistors, and inductors. Flicker noise may be characterized by a corner frequency FL, which is the point where the flicker noise is generally equal to white noise. Referring to FIGS. 1, 2, and 3, graphs of flicker noise over frequency is shown, together with the corner frequency FL.
Flicker noise becomes more pronounced with smaller device geometries and lower operating voltages. For higher frequency, lower voltage digital circuits with higher data rates flicker noise is a dominant cause of logic errors increasing the importance of accurate measurements of flicker noise. Flicker noise increases as the size of devices decreases and as the flicker noise increases so does the flicker noise bandwidth. While in the past flicker noise testing was done in the KHz frequency region, smaller devices producing increased flicker noise make it desirable that the flicker noise test system be capable of measuring the flicker noise at frequencies up to generally 30 MHz to more fully characterize the flicker noise spectrum. The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.